Editing Geographic information system (section)

====Secondary data capture====
{{further|Digitizing}}
The most common method of data creation is [[Digitizing|digitization]], where a [[hard copy]] map or survey plan is transferred into a digital medium through the use of a CAD program, and geo-referencing capabilities. With the wide availability of [[Orthophoto|ortho-rectified imagery]] (from satellites, aircraft, Helikites and UAVs), heads-up digitizing is becoming the main avenue through which geographic data is extracted. Heads-up digitizing involves the tracing of geographic data directly on top of the aerial imagery instead of by the traditional method of tracing the geographic form on a separate [[Graphics tablet|digitizing tablet]] (heads-down digitizing). Heads-down digitizing, or manual digitizing, uses a special magnetic pen, or stylus, that feeds information into a computer to create an identical, digital map. Some tablets use a mouse-like tool, called a puck, instead of a stylus.<ref>{{Cite journal|last=Puotinen|first=Marji|date=June 2009|title=A Primer of GIS: Fundamental Geographic and Cartographic Concepts - By Francis Harvey|journal=Geographical Research|volume=47|issue=2|pages=219–221|doi=10.1111/j.1745-5871.2009.00577.x|bibcode=2009GeoRs..47..219P |issn=1745-5863|doi-access=free}}</ref><ref name=":2">{{Cite web|title=Digitizing - GIS Wiki {{!}} The GIS Encyclopedia|url=http://wiki.gis.com/wiki/index.php/Digitizing|access-date=2021-01-29|website=wiki.gis.com}}</ref> The puck has a small window with cross-hairs which allows for greater precision and pinpointing map features. Though heads-up digitizing is more commonly used, heads-down digitizing is still useful for digitizing maps of poor quality.<ref name=":2" />

Existing data printed on paper or [[PET film (biaxially oriented)|PET film]] maps can be [[digitizer|digitized]] or scanned to produce digital data. A digitizer produces [[Vector graphics|vector]] data as an operator traces points, lines, and polygon boundaries from a map. [[Image scanner|Scanning]] a map results in raster data that could be further processed to produce vector data.

When data is captured, the user should consider if the data should be captured with either a relative accuracy or absolute accuracy, since this could not only influence how information will be interpreted but also the cost of data capture.

After entering data into a GIS, the data usually requires editing, to remove errors, or further processing. For vector data it must be made "topologically correct" before it can be used for some advanced analysis. For example, in a road network, lines must connect with nodes at an intersection. Errors such as undershoots and overshoots must also be removed. For scanned maps, blemishes on the source map may need to be removed from the resulting [[Raster graphics|raster]]. For example, a fleck of dirt might connect two lines that should not be connected.